Methods and Apparatus for Providing Data Transfer Control

ABSTRACT

A variety of advantageous mechanisms for improved data transfer control within a data processing system are described. A DMA controller is described which is implemented as a multiprocessing transfer engine supporting multiple transfer controllers which may work independently or in cooperation to carry out data transfers, with each transfer controller acting as an autonomous processor, fetching and dispatching DMA instructions to multiple execution units. In particular, mechanisms for initiating and controlling the sequence of data transfers are provided, as are processes for autonomously fetching DMA instructions which are decoded sequentially but executed in parallel. Dual transfer execution units within each transfer controller, together with independent transfer counters, are employed to allow decoupling of source and destination address generation and to allow multiple transfer instructions in one transfer execution unit to operate in parallel with a single transfer instruction in the other transfer unit. Improved flow control of data between a source and destination is provided through the use of special semaphore operations, signals and message synchronization which may be invoked explicitly using SIGNAL and WAIT type instructions or implicitly through the use of special “event-action” registers. Transfer controllers are also described which can cooperate to perform “DMA-to-DMA” transfers. Message-level synchronization can be used by transfer controllers to synchronize with each other.

RELATED APPLICATIONS

The present application is a divisional of U.S. Ser. No. 12/536,568filed Aug. 6, 2009 which is a divisional of U.S. Ser. No. 11/830,448filed Jul. 30, 2007 which is a continuation of U.S. Ser. No. 11/533,897filed Sep. 21, 2006 which is a continuation of U.S. Ser. No. 11/100,697filed Apr. 7, 2005, which is a divisional of U.S. Ser. No. 10/782,201filed Feb. 19, 2004, which is a continuation of U.S. Ser. No. 10/254,105filed Sep. 24, 2002, which is a continuation of U.S. Ser. No. 09/896,687filed Jun. 29, 2001, which is a divisional of U.S. Ser. No. 09/471,217filed Dec. 23, 1999, which claims the benefit of U.S. ProvisionalApplication Ser. No. 60/113,555 filed Dec. 23, 1998.

FIELD OF THE INVENTION

The present invention relates generally to improvements in arrayprocessing, and more particularly to advantageous techniques forproviding improved data transfer control.

BACKGROUND OF THE INVENTION

Various prior art techniques exist for the transfer of data betweensystem memories or between system memories and input/output (I/O)devices. FIG. 1 shows a conventional data processing system 100comprising a host uniprocessor 110, processor local memory 120, I/Odevices 130 and 140, a system memory 150 which is usually a largermemory store than the processor local memory and having longer accesslatency, and a direct memory access (DMA) controller 160.

The DMA controller 160 provides a means for transferring data betweenprocessor local memory and system memory or I/O devices concurrent withuniprocessor execution. DMA controllers are sometimes referred to as I/Oprocessors or transfer processors in the literature. System performanceis improved since the Host uniprocessor can perform computations whilethe DMA controller is transferring new input data to the processor localmemory and transferring result data to output devices or the systemmemory. A data transfer is typically specified with the followingminimum set of parameters: source address, destination address, andnumber of data elements to transfer. Addresses are interpreted by thesystem hardware and uniquely specify I/O devices or memory locationsfrom which data must be read or to which data must be written. Sometimesadditional parameters are provided such as element size. In addition,some means of initiating the data transfer are provided, and alsoprovided is a means for the DMA controller to notify the hostuniprocessor when the transfer is complete. In some conventional DMAcontrollers, transfer initiation may be carried out by programmingspecific registers within the DMA controller. Others are designed tofetch their own “transfer descriptors” which might be stored in one ofthe system memories. These descriptors contain the information requiredto carry out a specific transfer. In the latter case, the DMA controlleris provided a starting address from which to fetch transfer descriptorsand there must be some means for controlling the fetch operation.End-of-transfer (EOT) notification in conventional DMA controllers maytake the form of signaling the host uniprocessor so that it generates aninterrupt which may then be handled by an interrupt service routine. Inother notification approaches, the DMA controller writes a notificationvalue to a specified memory location which is accessible by the hostuniprocessor. One of the limitations of conventional DMA controllers isthat address generation capabilities for the data source and datadestination are often constrained to be the same. For example, when onlya source address, destination address and a transfer count arespecified, the implied data access pattern is block-oriented, that is, asequence of data words from contiguous addresses starting with thesource address is copied to a sequence of contiguous addresses startingat the destination address. Another limitation of conventional DMAcontrollers is the overhead required to manage the DMA controller interms of transfer initiation, data flow control during a transfer, andhandling EOT notification.

With the advent of the ManArray architecture, it has been recognizedthat it will be advantageous to have improved techniques for carryingout such functions tailored to this new architecture.

SUMMARY OF THE INVENTION

As described in detail below, the present invention addresses a varietyof advantageous methods and apparatus for improved data transfer controlwithin a data processing system. In particular, improved mechanisms areprovided for initiating and controlling the sequence of data transfers;decoupling source and destination address generation through the use ofindependent specification of source and destination transfer descriptors(hereafter referred to as “DMA instructions” to distinguish them from aspecific type of instruction called a “transfer instruction” whichperforms the data movement operation); executing multiple “source”transfer instructions for each “destination” transfer instruction, ormultiple “destination” transfer instructions for each “source” transferinstruction; intra-transfer control of the flow of data (control thatoccurs while a transfer is in progress); EOT notification; andsynchronizing of data flow with a compute processor and with one or morecontrol processors through the use of SIGNAL and WAIT operations onsemaphores.

Additionally, the present invention provides a DMA controllerimplemented as a multiprocessor consisting of multiple transfercontrollers each supporting its own instruction thread. It allowscooperation between transfer controllers seen in the DMA-to-DMA methodaddressed further below. It addresses single-thread of control of dualtransfer units or execution units. Execution control of a transferinstruction may advantageously be based on a flag in the instructionitself. Multiple instructions may execute in one unit while a singleinstruction executes in the other. Independent transfer counters for CTUand STU are provided. Conditional SIGNAL instructions which can sendmessages on control bus, interrupts or update semaphores areadvantageously provided, as is a conditional WAIT instruction which isexecuted based on the state of a semaphore. When a wait conditionbecomes false, this semaphore is updated according to instruction.Further aspects include the use of transfer conditions in branch, SIGNALand WAIT instructions (STUEOT, CTUEOT, notSTUEOT, notCTUEOT). Further,the use of semaphores is addressed as the basis for conditionalexecution. A generalization of these techniques allows dual-CTU ordual-STU transfer controllers. A dual-CTU transfer controller might beused to perform DMA transfers from one cluster's DMA bus to anothercluster's DMA bus. Further, a restart capability based on RESTARTcommands, Load-transfer-count-and-restart commands, or a semaphoreupdate from an SCB master is addressed.

These and other advantages of the present invention will be apparentfrom the drawings and the Detailed Description which follow.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a conventional data processing system with a DMAcontroller to support data transfers concurrent with host processorcomputation;

FIG. 2 shows a ManArray DSP with a DMA controller in a system inaccordance with the present invention;

FIG. 3 illustrates a DMA controller implemented as a multiprocessor,showing two transfer controllers, bus connections to a system memory, PEmemories and a control bus;

FIG. 4A shows a single transfer controller comprising 4 primaryexecution units, bus connections and FIFO buffers;

FIG. 4B shows paths over which DMA instructions may be fetched;

FIG. 4C illustrates transfer controller instruction types;

FIG. 4D shows an exemplary transfer program counter (TPC) register;

FIG. 4E shows an exemplary wait program counter (WAITPC) register;

FIG. 4F shows exemplary commands and addresses for a presently preferredembodiment of the present invention;

FIG. 5A shows how TPC and WAITPC register can be used to controlinstruction fetching in accordance with the present invention;

FIG. 5B shows an exemplary LOCK register used for mutual exclusiveaccess to the WAITPC register;

FIG. 5C shows an exemplary link program counter (LINKPC) register;

FIG. 5D illustrates conditions which may be used for branchinstructions, and SIGNAL and WAIT instructions;

FIG. 5E shows an exemplary format for semaphore registers for storing8-bit semaphores;

FIG. 6 shows a general format of a transfer instruction type;

FIG. 7 shows a logical view of a top-level transfer controller statemachine for use in conjunction with the present invention;

FIG. 8A shows a transfer unit state machine (either STU or CTU);

FIG. 8B shows exemplary pseudo-code for a simple inbound block transferwith execute flag active;

FIG. 8C shows exemplary pseudo-code for a simple inbound block transferwith execute flag inactive;

FIG. 9A shows separate transfer counters and data paths for STU and CTUcontrol logic;

FIG. 9B shows an exemplary initial transfer count register;

FIG. 9C shows an exemplary current transfer count register;

FIG. 9D shows an exemplary data “gather” instruction sequenceillustrating how a single STU instruction can operate with multiple CTUinstructions from the same instruction thread;

FIG. 9E shows an example of a data “scatter” instruction sequenceillustrating how a single CTU instruction can operate with multiple STUinstructions from the same instruction thread;

FIG. 9F shows an exemplary format for an external signal register;

FIG. 9G illustrates an event action register 0;

FIG. 9H shows an exemplary format for a SIGNAL instruction;

FIG. 9I describes an event action register 1;

FIG. 9J shows an exemplary format for general registers;

FIG. 10A shows an event control unit;

FIG. 10B shows an exemplary format for a WAIT instruction; and

FIG. 10C shows an exemplary instruction sequence which allowsindependent flow control of data transfer by two host processors.

DETAILED DESCRIPTION

Further details of a presently preferred ManArray core, architecture,and instructions for use in conjunction with the present invention arefound in U.S. patent application Ser. No. 08/885,310 filed Jun. 30,1997, U.S. patent application Ser. No. 08/949,122 filed Oct. 10, 1997,U.S. patent application Ser. No. 09/169,255 filed Oct. 9, 1998, U.S.patent application Ser. No. 09/169,256 filed Oct. 9, 1998, U.S. patentapplication Ser. No. 09/169,072 filed Oct. 9, 1998, U.S. patentapplication Ser. No. 09/187,539 filed Nov. 6, 1998, U.S. patentapplication Ser. No. 09/205,558 filed Dec. 4, 1998, U.S. patentapplication Ser. No. 09/215,081 filed Dec. 18, 1998, U.S. patentapplication Ser. No. 09/228,374 filed Jan. 12, 1999 and entitled“Methods and Apparatus to Dynamically Reconfigure the InstructionPipeline of an Indirect Very Long Instruction Word Scalable Processor”,U.S. patent application Ser. No. 09/238,446 filed Jan. 28, 1999, U.S.patent application Ser. No. 09/267,570 filed Mar. 12, 1999, U.S. patentapplication Ser. No. 09/337,839 filed Jun. 22, 1999, U.S. patentapplication Ser. No. 09/350,191 filed Jul. 9, 1999, U.S. patentapplication Ser. No. 09/422,015 filed Oct. 21, 1999 entitled “Methodsand Apparatus for Abbreviated Instruction and Configurable ProcessorArchitecture”, U.S. patent application Ser. No. 09/432,705 filed Nov. 2,1999 entitled “Methods and Apparatus for Improved Motion Estimation forVideo Encoding”, U.S. patent application Ser. No. 09/482,372 entitled“Methods and Apparatus for Providing Direct Memory Access Control” filedDec. 23, 1999, as well as, Provisional Application Ser. No. 60/113,637entitled “Methods and Apparatus for Providing Direct Memory Access (DMA)Engine” filed Dec. 23, 1998, Provisional Application Ser. No. 60/113,555entitled “Methods and Apparatus Providing Transfer Control” filed Dec.23, 1998, Provisional Application Ser. No. 60/139,946 entitled “Methodsand Apparatus for Data Dependent Address Operations and EfficientVariable Length Code Decoding in a VLIW Processor” filed Jun. 18, 1999,Provisional Application Ser. No. 60/140,245 entitled “Methods andApparatus for Generalized Event Detection and Action Specification in aProcessor” filed Jun. 21, 1999, Provisional Application Ser. No.60/140,163 entitled “Methods and Apparatus for Improved Efficiency inPipeline Simulation and Emulation” filed Jun. 21, 1999, ProvisionalApplication Ser. No. 60/140,162 entitled “Methods and Apparatus forInitiating and Re-Synchronizing Multi-Cycle SIMD Instructions” filedJun. 21, 1999, Provisional Application Ser. No. 60/140,244 entitled“Methods and Apparatus for Providing One-By-One Manifold Array (1×1ManArray) Program Context Control” filed Jun. 21, 1999, ProvisionalApplication Ser. No. 60/140,325 entitled “Methods and Apparatus forEstablishing Port Priority Function in a VLIW Processor” filed Jun. 21,1999, Provisional Application Ser. No. 60/140,425 entitled “Methods andApparatus for Parallel Processing Utilizing a Manifold Array (ManArray)Architecture and Instruction Syntax” filed Jun. 22, 1999, ProvisionalApplication Ser. No. 60/165,337 entitled “Efficient Cosine TransformImplementations on the ManArray Architecture” filed Nov. 12, 1999, andProvisional Application Ser. No. 60/171,911 entitled “Methods andApparatus for Loading a Very Long Instruction Word Memory” filed Dec.23, 1999, respectively, all of which are assigned to the assignee of thepresent invention and incorporated by reference herein in theirentirety.

The following definitions of terms are provided as background for thediscussion of the invention which follows below:

A “transfer” refers to the movement of one or more units of data from asource device (either I/O or memory) to a destination device (I/O ormemory).

A data “source” or “destination” refers to a device from which data maybe read or to which data may be written which is characterized by acontiguous sequence of one or more addresses, each of which isassociated with a data storage element of some unit size. For some datasources and destinations there is a many-to-one mapping of addresses todata element storage locations. For example, an I/O device may beaccessed using one of many addresses in a range of addresses, yet forany of them it will perform the same read/write operation.

A “data access pattern” is a sequence of data source or destinationaddresses whose relationship to each other is periodic. For example, thesequence of addresses 0, 1, 2, 4, 5, 6, 8, 9, 10, . . . etc. is a dataaccess pattern. If we look at the differences between successiveaddresses, we find: 1,1,2, 1,1,2, 1,1,2, . . . etc. Every three elementsthe pattern repeats.

“EOT” means “end-of-transfer” and refers to the state when a transferexecution unit (described in the following text) has completed its mostrecent transfer instruction by transferring the number of elementsspecified by the instruction's transfer count field.

As used herein, an “overrun at the source” of a transfer occurs when theproducer of data over-writes data that the DMA controller has not yetread. An “overrun at the destination” of a transfer occurs when the DMAcontroller overwrites data that has not yet been processed by a consumerof data. An “underrun at the source” occurs when the DMA controllerattempts to read data that has not yet been written by the producer, andan “underrun at the destination” occurs when the consumer task attemptsto read and process data that the DMA controller has not yet written.

The term “host processor” as used in the following discussion refers toany processor or device that can write control commands and read statusfrom the DMA controller and/or that can respond to DMA controllermessages and signals. In general a host processor interacts with the DMAcontroller to control and synchronize the flow of data between devicesand memories in the system in such a way as to avoid overrun andunderrun conditions at the sources and destinations of data transfers.

FIG. 2 shows an exemplary system 200 illustrating the context in which aManArray DMA controller 201, in accordance with the present invention,resides. The DMA controller 201 accesses processor local memories 210,211, 212, 213, 214 and 215 via the DMA Bus 202, 202 ₁, 202 ₂, 202 ₃, 202₄, and 202 ₅ and the memory interface units 205, 206, 207, 208 and 209to which it is connected. A ManArray DSP 203 also connects to its localmemories 210-215 via memory interface units 205-209. Further details ofa presently preferred DSP 203 are found in the above incorporated byreference applications.

In the representative system, the DMA controller also connects to twosystem busses, a system control bus (SCB) 235 and a system data bus(SDB) 240. The DMA controller is designed to transfer data betweendevices on the SDB 240, such as system memory 250 and the DSP 203 localmemories 210-215. The SCB 235 is used by an SCB master such as DSP 203or a host control processor (HCP) 245 to program the DMA controller 201(read and write addresses and registers to initiate control operationsand read status). The SCB 235 is also used by the DMA Controller 201 tosend synchronization messages to other SCB bus slaves such as the DSPcontrol registers 225 and the Host I/O block 255. Some registers inthese slaves can be polled by the DSP and HCP to receive status from theDMA. Alternatively, DMA writes to some of these slave addresses can beprogrammed to cause interrupts to the DSP and/or HCP allowing DMAcontroller messages to be handled by interrupt service routines.

FIG. 3 shows a system 300 which illustrates a DMA controller 301 whichmay suitably be a multiprocessor specialized to carry out data transfersutilizing one or more transfer controller units 302 and 303. Eachtransfer controller can operate as an independent processor or worktogether with other transfer controllers to carry out data transfers.The DMA busses 305 and 310 provide, in the presently preferredembodiment, independent data paths to local memories 320, 321, 322, 323,324, 325 for each transfer controller 302 and 303. In addition, eachtransfer controller is connected to an SDB 350 and to an SCB 330. Eachtransfer controller operates as a bus master and a bus slave on both theSCB and SDB. As a bus slave on the SCB, a transfer controller may beaccessed by other SCB bus masters in order to read its internal state orissue control commands. As a bus master on the SCB, a transfercontroller can send synchronization messages to other SCB bus slaves. Asa bus master on the SDB, a transfer controller performs data reads andwrites from or to system memory or I/O devices which are bus slaves onthe SDB. As a bus slave on the SDB, a transfer controller can cooperatewith another SDB bus master in a “slave mode” allowing the bus master toread or write data directly from or to its data FIFOs (as discussedfurther below). It may be noted that the DMA Busses 305 and 310, the SDB350 and the SCB 330 may be implemented in different ways, for example,with varying bus widths, protocols, or the like, consistent with theteachings of the current invention.

FIG. 4A shows a system 400 having a single transfer controller 401comprising a set of execution units including an instruction controlunit (ICU) 440, a system transfer unit (STU) 402, a core transfer unit(CTU) 408 and an event control unit (ECU) 460. An inbound data queue(IDQ) 405 is a data FIFO which is written with data from the SDB 470under control of the STU 402. Data to be sent to core memories 430, orsent to the ICU 440 in the case of instruction fetches is read from theIDQ 405 under control of the CTU 408. An outbound data queue (ODQ) 406is a data FIFO which is written with data from the DMA busses 425 undercontrol of the CTU 408, to be sent to an SDB 470 device or memory underthe control of the STU 402. The CTU 408 may also read DMA instructionsfrom a memory attached to the DMA bus. These instructions are thenforwarded to the ICU 440 for initial decode. The ECU 460 receives signalinputs from external devices 465, commands from the SCB 450 andinstruction data from the ICU 440. It generates output signals 435, 436and 437 which may be used to generate interrupts on host controlprocessors within the system, and can act as a bus master on the SCB 450to send synchronization messages to SCB bus slaves.

Transfer Sequence Control

Each transfer controller within a ManArray DMA controller is designed tofetch its own stream of DMA instructions. DMA instructions may befetched from memories located on any of the busses which are connectedto the transfer controller: DMA busses, SDB or SCB. FIG. 4B shows asystem 475 illustrating data paths from which instructions may befetched. A transfer controller 476 can fetch DMA instructions frommemories on the DMA Bus 478 and provide them on a path 484 to the ICU486 under the control of the CTU 481. A second path 488 allows DMAinstructions to be fetched from the SDB 480 under the control of the STU482 through the IDQ 489 under the control of the CTU 481 and thenforwarded to the ICU 486. A third path allows instructions to be fetchedfrom memories or devices on the SCB 479 on a data path 492 through theECU 494 (which controls the SCB master interface) and then forwarded tothe ICU 486. After receiving instructions, the ICU 486 decodes the firstinstruction word of each instruction, determines the number of remaininginstruction words and forwards the control signals and additionalinstruction words to the execution units CTU 481, STU 482 and ECU 494via an internal instruction bus 495. The ODQ 490 is not used forinstruction fetch purposes.

DMA instructions are of five basic types: transfer; branch; load;synchronization; and state control. The branch, load, synchronization,and state control types of instructions are collectively referred to as“control instructions”, and distinguished from the transfer instructionswhich actually perform data transfers. DMA instructions are typically ofmulti-word length and require a variable number of cycles to executealthough several control instructions require only a single word tospecify. DMA instructions will be described in greater detail below.FIG. 4C is a table 455 which shows a set of instruction types 456, listtheir operations 457 and briefly describes their functions 458 in apresently preferred embodiment of the invention. In table 455, “cc”indicates that instruction execution depends on a condition specified inthe instruction.

Two registers are to support the fetching of instructions: a transferprogram counter (TPC) register 459 of FIG. 4D, and a wait programcounter (WAITPC) 462 of FIG. 4E. In a preferred embodiment, theseregisters have a sufficient number of bits (e.g. 32) to address allmemories which may contain instructions. The TPC contains the address ofthe next instruction word to be fetched and decoded. After fetching acomplete instruction and updating the TPC, the control logic comparesthe value of TPC with the value stored in WAITPC. If TPC and WAITPC areequal, then the fetching and decoding of instructions is suspended. Inthe preferred embodiment, a block of instruction words is fetched into alocal cache from which they are read and decoded, but this is only oneof many methods to decrease instruction fetch latency for subsequentinstructions. At powerup or after a reset command or signal is receivedby a transfer controller, TPC and WAITPC are set to the same value. Acommand address is provided called the INITPC address 463 FIG. 4F which,when written with a DMA instruction address value, updates both the TPCand WAITPC registers with the value, allowing an instruction startaddress to be specified without initiating the fetching of instructions.Writing a new value to either TPC or WAITPC and thereby making the twodifferent will cause instruction fetching to proceed.

FIG. 5A shows a sequence of DMA instructions 500. The contents of TPC550 address the first instruction 510 in the sequence, which hasmultiple words 511 and 512 as parameters. The address in WAITPC register560 points to the word 565 immediately following the last validinstruction word 540. After fetching, decoding and executing theremaining instructions up to the address in WAITPC (520, 521, 522, 530,531, 540), TPC 550 will become equal to WAITPC and instruction fetchingand decode will suspend. Instructions may be added to memory locationsfollowing the address in WAITPC as shown with the TSI 570 and TCI 580instructions. In order to resume fetching instructions, either the TPCor the WAITPC register must be changed so that TPC no longer matchesWAITPC. In a presently preferred embodiment, WAITPC must contain theaddress of the first word of an instruction for it to suspend fetchingwhen a match occurs since the comparison only takes place prior tostarting the fetch and decode of a new instruction. This choice isimplementation specific and is made to simplify the logic of multi-wordinstruction decode. Also, there are some instructions which, byexecuting, cause fetching to be suspended, such as the WAIT instruction540 in FIG. 5A.

Mechanism for Exclusive Access to WAITPC

If there are multiple host processors which wish to update or addinstructions to the DMA instruction list, then it is necessary that someform of mutual exclusive access to the WAITPC register be maintained. Ahardware support means for this mutual exclusion is provided through theuse of a LOCK register 575 illustrated in FIG. 5B, and a set of LOCKIDread-only addresses 464 of FIG. 4F which are recognized by the transfercontroller's SCB slave logic. The 8 read-addresses, or LOCKID addresses464, are set aside in the transfer controller's command/address spaceand are visible to SCB bus masters. They are used in the followingmanner:

Each host processor which needs to update the transfer controller's DMAinstruction list is assigned one of the 8 unique LOCKID addresses.

When a host processor wishes to add instructions ahead of the currentWAITPC value, it reads from its own LOCKID address. The transfercontroller returns the value of the “locked” bit 576 of the LOCKregister 575 of FIG. 5B.

If the value returned is 0, then no other host processor currently ownsthe lock. The processor becomes the new owner of the “lock” on theWAITPC register and may now append instructions freely, starting at thecurrent WAITPC address. When a host processor becomes owner of the lock,the “locked” bit of the LOCK register is set to “1”, and the lower 3bits of the host processor's LOCKID address are written to bits[2-0] ofthe LOCK register 575.

If the value returned is 1 then another host processor currently ownsthe lock on WAITPC, and the requesting host processor must continuepolling its LOCKID address until a value of 0 is returned, indicatingthat it has received ownership of the lock on WAITPC.

When a host processor which owns the lock has finished updating theinstruction list, it writes a new value to WAITPC pointing to the nextinstruction location immediately after the last instruction added. Theact of writing to the WAITPC clears the “locked” flag in the LOCKregister, making it available to another processor.

The hardware does not prevent write access to the WAITPC register, butonly provides a semaphore mechanism to facilitate software scheduling ofthe WAITPC (i.e. DMA instruction list) resource.

The LOCK register is a read-only register that returns the identity ofthe last (or current) owner of the lock and the status of the “locked”bit 576 of FIG. 5B.

It will be evident that the choice of the number of lock addresses to beassigned is arbitrary and the method and apparatus can be extended orreduced to support more or fewer SCB masters.

Branch Instructions

Instruction sequencing can also be controlled by executing branch-typeinstructions. The transfer controller supports five types of branchinstructions 439 as shown in FIG. 4C: jump-relative, jump-absolute,call-relative, call-absolute, and return. Jump-relative loads the TPCwith the sum of TPC and an immediate offset value contained in theinstruction. Jump-absolute loads TPC with an immediate value containedin the instruction. Call-relative operates the same as jump-relative,except that before loading TPC with the new value, the old value whichpoints to the address immediately following the CALL instruction iscopied to a link counter register 577 called LINKPC shown in FIG. 5C.Call-absolute operates the same as jump-absolute, except a copy of theold TPC is stored in LINKPC prior to updating TPC. The returninstruction RET copies the value of LINKPC to TPC. Instruction fetchthen resumes from the updated TPC address as long as TPC is not equal toWAITPC.

All branch instructions are conditional. FIG. 5D shows a list 578 of thecondition specifiers which may be tested to determine whether a branchshould be taken or not. One of the condition specifiers is “Always”,meaning that the branch is always taken making it unconditional.Condition specifiers are both arithmetic (Equal, NotEqual, Higher,HigherOrEqual, Lower, LowerOrEqual, GreaterOrEqual, Greater,LessOrEqual, Less) and non-arithmetic (CTUeot, STUeot, NotCTUeot,NotSTUeot, Always) as shown in FIG. 5D. In order to determine the truthvalue of an arithmetic condition a semaphore register (such as one ofregisters S0, S1, S2 or S3 579 shown in FIG. 5E which illustrates thepresently preferred embodiment) specified in the instruction is comparedwith zero. If the relationship between the semaphore value and zero isthe same as that specified by the condition specifier (e.g. “Greater”,or “Equal”), then the branch condition is TRUE. Otherwise, it is FALSE.If the condition is TRUE, the branch is taken, and an optional update tothe semaphore is made (increment, decrement, clear to zero, or nochange). If the branch is FALSE, the branch instruction is treated as anNOP (“no-operation”). It is ignored and no update to the semaphore isperformed.

For example, the instruction, jmp.GT S0—, newlocation, comparessemaphore register S0 to zero. If it is greater than zero (“GT”), thenthe branch to “newlocation” occurs (the address of “newlocation” isloaded into TPC and the next instruction is fetched from there). Inaddition, the semaphore S0 is decremented by 1 as a side-effect (“S0—”).If the register S0 is less than or equal to zero (S0 is treated as asigned two's complement number), then the branch is not taken and nodecrement of S0 occurs.

Four of the five non-arithmetic conditions (CTUeot, STUeot, NotCTUeotand NotSTUeot) allow branches to be taken or not, depending on transferunit status. These conditions are useful for controlling the instructionsequence when instructions are fetched after a transfer has completed.Since either the STU or the CTU can finish processing an instructionbefore the other if their transfer counts differ, it is sometimes usefulto conditionally branch based on which unit completes first.

Instruction Decode, Dispatch and Execute

Referring again to system 400 of FIG. 4A, transfer-type instructions aredispatched by the ICU 440 for further decode execution by the STU 402and the CTU 408. Transfer instructions have the property that they arefetched and decoded sequentially, in order to load transfer parametersinto the appropriate execution unit, but are executed concurrently. Thecontrol mechanism for initiating execution of transfer instructions is aflag bit contained in the instruction itself, and is described below.

A “transfer-system-inbound” or TSI instruction moves data from the SDB470 to the IDQ 405 and is executed by the STU. A “transfer-core-inbound”or TCI instruction moves data from the IDQ 405 to the DMA Bus 425 and isexecuted by the CTU. A “transfer-core-outbound” or TCO instruction movesdata from the DMA Bus 425 to the ODQ 406 and is executed by the CTU. A“transfer-system-outbound” or TSO instruction moves data from the ODQ406 to the SDB 470 and is executed by the STU. Two transfer instructionsare required to move data between an SDB system memory and one or moreSP or PE local memories on the DMA Bus, and both instructions areexecuted concurrently: a (TSI, TCI) pair or a (TSO, TCO) pair. Theaddress parameter of STU transfer instructions (TSI and TSO) refers toaddresses on the SDB while the address parameter of CTU transferinstructions refers to addresses on the DMA Bus to PE and SP localmemories.

FIG. 6 shows an exemplary instruction format 600 for transferinstructions. A base opcode field 601 indicates that the instruction isof the transfer type. A C/S field 610 indicates the transfer unit (CTUor STU) and an I/O field 620 indicates whether the transfer direction isinbound or outbound. A data type field 630 indicates the size of eachelement transferred and an address mode 640 refers to the data accesspattern which must be generated by the transfer unit. Transfer count 660indicates the number of data elements of size “data type” which are tobe transferred to or from the target memory/device before EOT occurs forthat unit. An address parameter 670 specifies the starting address forthe transfer, and other parameters 680 follow the address word of theinstruction (some addressing modes require additional parameters). The“X” (execute) field 650 is a field which, when set to “1” indicates a“start transfer” event, that is, the transfer should start immediatelyafter loading the transfer instruction. When the “X” field is “0”, thenthe parameters are loaded into the specified unit, but instructionfetch/decode continues until a “start transfer” event occurs.

FIG. 7 shows global states 700 within which the transfer controlleroperates. A transfer controller RESET event, such as powerup orreceiving a RESET command or signal, causes a logic transition T0 701 toadvance to a CHECKTPC state 710 in which the TPC is compared withWAITPC. Since TPC is equal to WAITPC after reset, no instructionfetching occurs. When WAITPC or TPC is updated so that TPC is not equalto WAITPC, transition T1 715 occurs, placing the transfer controller ina FETCH state 720. After an instruction word is fetched, T2 transition725 to DECODE state 730 occurs. If the instruction is multiple words,then transitions T10 786 to FETCH 720 is followed by transitions T2 725to DECODE 730 occur until all instruction words have been processed.With each word fetched, the TPC is incremented by one instruction wordaddress. If the instruction is a control type instruction, transition T3775 to EXEC CONTROL 760 occurs and the instruction action is performed,followed by a transition T12 785 back to CHECKTPC 710.

Executing a WAIT type instruction (with a TRUE condition—discussedfurther below) causes the transfer controller to take transition T5 765to WAIT state 755. When the wait condition becomes FALSE, transition T11766 returning to EXEC CONTROL 760 occurs to complete the WAITinstruction execution, followed by a transition T12 785 back to CHECKTPC710. When in the DECODE state 730 and a transfer type instruction hasbeen decoded, and a start transfer event is detected (“X” field in theinstruction is “1”), the transition T4 735 to EXEC TRANSFER 740 occurs.The transfer continues until an EOT (end-of-transfer) condition isdetected, at which time a transition T6 795 back to CHECKTPC 710 occurs.Transitions T7 745 and T9 796 occur when a “restart transfer” event isdetected in the WAIT state 755 and CHECKTPC state 710 respectively. Whena restart event is detected while in the WAIT state and transition T7occurs to the EXEC TRANSFER 740 state, when the transfer is complete(either STU or CTU reaches EOT), then transition T8 back to the WAIT 755state occurs. Restart transfer events are further described below.

While the transfer controller operates in one of the global states 700of FIG. 7, FIG. 8 shows the sub-states 800 in which the transfer units(STU and CTU) operate. The transfer units are driven by the ICUinstruction dispatcher 440 and by event monitoring logic in the ECU 460of FIG. 4A. After a RESET event, transition T0 810 places the transferunit into the INACTIVE state 815. In this state, neither a “starttransfer event” nor a “restart transfer event” can cause the transferunit to begin a transfer sequence since transfer parameters areconsidered invalid. When a transfer unit detects new transfer parametersare being loaded, transition T1 820 takes the unit to the DECODE state825. After loading all transfer instruction parameters, if the execute“X” flag of the instruction is not “1”, then transition T2 830 takes thetransfer unit to the IDLE state 840. If the “X” flag is “1” (“starttransfer”), then transition T5 855 places the unit into the TRANSFERstate 850. When the unit detects its EOT condition, transition T4 835places the unit back into the IDLE state 840. If a “restart transfer”event is detected while in the IDLE state 840, transition T3 845 placesthe unit back into the TRANSFER state 850. If a CLEAR command from anSCB bus master is received in any state, the transfer units parametersare invalidated and the logic makes the transition T7 860 to theINACTIVE state 815.

As addressed previously, for most transfers, two transfer instructionsare required to move data from a source memory or device to adestination memory or device, one executing in the CTU and one in theSTU. FIG. 8B shows an instruction sequence 875 to perform a simple blocktransfer. The “.x” on the tci.block.x instruction indicates immediateinitiation of the transfer after decoding both instructions. FIG. 8Cshows an instruction sequence 885 for a second instruction is the sameas sequence 875 only without the “.x” (execute) directive. In this case,the transfer is not started, but the following WAIT instruction isfetched and executed. In other words, the logic waits for an external“start event” to occur, either a RESTART command or a RESUME which willcause instruction fetching to continue. These commands are shown in FIG.4F. Note that in this example, both transfer counts are the same. One ofthe features of the present invention is that the STU and CTU operatewith independent transfer counters, making it possible to executemultiple transfer instructions in one transfer unit, while the other isprocessing a single transfer instruction. This result is achieved byspecifying a sequence of instructions in which the transfer counts aredifferent in each transfer unit.

FIG. 9A illustrates a separate transfer counter logic 900. Blocks 985and 910 are parts of the STU and CTU logic respectively. CTU controllogic 940 controls the updates of counters 915, 920, 935, and generationof a CTU EOT 945. STU control logic 955 controls the updates of counters960, 975, 980, and generation of an STU EOT 950. An instruction bus 901feeds both transfer units. When one of the transfer units is decoding atransfer instruction as specified by the C/S field 610 of the transferinstruction 600 of FIG. 6, the transfer count specified in theinstruction is copied to its initial transfer count register, ISTC 980or ICTC 915 and current transfer count register STC 975 or CTC 920through multiplexers 972 and 922, respectively. The ISTC and ICTCregisters retain the initial transfer count as loaded by the lasttransfer instruction, or by a direct command from an SCB bus master.When a transfer is started, either by a “start transfer” indicator in atransfer instruction, or by another restart event, a minimum transfercount value is calculated by minimum calculator 905, as the minimum ofSTC 975 and CTC 920. This value becomes the minimum count value storedby counters MinSTC 960 and MinCTC 935, and is also subtracted bysubtractors 970 and 925 from both current transfer count values STC 975and CTC 920 and then stored back in the current transfer count registersthrough multiplexers 972 and 922. The MinSTC and MinCTC counters 965 and930 are decremented once for each data element transferred by theirrespective transfer units. The minimum transfer count value is used todetermine when an EOT condition has occurred, either CTU EOT 945 or STUEOT 950. Since the minimum of the two current transfer count values isalways used as the common transfer count, at least one of the transferunits will transfer its entire count of data elements and reach an EOTcondition, 945 or 950. When either transfer unit reaches an EOTcondition, instruction fetch and decode is reenabled in the ICU, and theother unit retains its last count value in its current transfer countregister, and its last access address so that if restarted, it willcontinue accessing data from where it left off. In the presentlypreferred embodiment, the initial transfer count values ISTC and ICTC980 and 915 may be read from a single register 986, ITCNT illustrated inFIG. 9B, and the current transfer count values may also be read from asingle register 987, TCNT illustrated in FIG. 9C.

FIG. 9D shows a DMA instruction sequence 988 that performs a singleoutbound STU transfer (from ODQ to system memory) while processing fouroutbound CTU transfer instructions (from PE local memories to the ODQ).Each of the four TCO transfers specifies an immediate execute. This hasthe effect of restarting the STU from where it left off (in terms of itstransfer count and last address accessed). The TSO (STU instruction)will have the effect of merging the data read by each of the four CTUtransfer instructions into a single block in System memory.

FIG. 9E shows a similar DMA instruction sequence 989, only now themultiple TSO instructions perform a scatter of the data read by thesingle TCO instruction. It will be recognized that instructions 988 and989 are only examples to show the flexibility of the present controlmethod.

Synchronizing A Host Processor (or Processors) with Data Transfer

In many applications, synchronization of host processing with datatransfer requires the following:

The transfer engine cannot be allowed to overtake the producer of data(underrun), and the data must be transferred before the produceroverwrites a region with valid but un-transferred data with new data(overrun). In other words, underrun and overrun conditions at the sourcemust be avoided.

Data transferred to the destination cannot overwrite unprocessed data(overrun), and the consumer of data can't be allowed to process invaliddata (i.e. a region of data that has not been updated by the transferengine). In other words, overrun and underrun at the destination must beavoided.

The control necessary to prevent underflow and overflow at the sourceand destination respectively should incur minimal overhead in the sourceand destination processors, and to a lesser extent the transfer enginewhose function is to hide transfer latency.

There are several synchronization mechanisms available which allow theserequirements to be met for each transfer controller. These mechanismswill be described by the direction of control flow, eitherhost-processor-to-transfer controller or transfer controller-to-hostprocessor where, for example, host-processor may refer to either the DSP203 or host control processor 245 of FIG. 2 or both.

Once a transfer has been started there must be some means for the hostprocessor to know when the transfer has completed or reached some “pointof interest”. These “points of interest” correspond to internal transferconditions which may be checked and which may then be used to generatesignaling actions back to the host processor or processors. Eachtransfer controller tracks the following internal conditions:

When TPC=WAITPC

When CTU has transferred the requested number of elements (CTU EOT)

When STU has transferred the requested number of elements (STU EOT)

When both CTU and STU have transferred the requested number of elements(CTU EOT AND STU EOT)

The “TPC=WAITPC” condition is checked during the CHECKTPC state 710 ofFIG. 7 and causes fetching to pause while the condition is true. Aspreviously stated, while in the EXEC TRANSFER state 740 a transfercontroller uses two transfer counters, the system transfer count (STC)and the core transfer count (CTC). The STC contains the number of dataelements to be transferred from (inbound) or to (outbound) the SDB. TheCTC contains the number of data elements to be transferred from(outbound) or to (inbound) the DMA Bus.

The main criteria for determining when an end-of-transfer (EOT)condition has occurred is that one of the transfer counters has reachedzero AND all data in the transfer path has been flushed to thedestination (FIFOs are empty, etc.). When an EOT condition is detectedthe transfer controller transitions to the CHECKTPC state 710, andproceeds to fetch and decode more instructions if TPC and WAITPC are notequal. The manner in which STC and CTC are decremented and EOT isdetermined depends on whether the transfer is inbound or outbound.

For outbound transfers, an EOT condition occurs when (STC reaches zeroOR CTC reaches zero) AND the ODQ FIFO is empty AND the SDB bus master isidle.

For inbound transfers, an EOT condition occurs when (STC reaches zero ORCTC reaches zero) AND the IDQ FIFO is empty AND the all data has beenwritten to the DSP local memory.

These conditions ensure that when the transfer controller signals that atransfer is complete, the data is actually valid for a host processor,and data coherence is maintained.

Host processors can communicate with the transfer controller usingeither commands (writes to special addresses), register updates (writeswith specific data), or discrete signals (usually from an I/O block). Inaddition, host processors can update the transfer controllersinstruction flow by using the WAITPC register to break transfer programsinto blocks of transfers. Multiple hosts can use the same DMA transfercontroller, updating its instruction stream by using the LOCKID registerand associated command addresses to implement mutually exclusive accessto the WAITPC. Semaphore commands may be used to both signal and wait ona semaphore, see command INCS0 491 in table 496 of exemplary commands,associated addresses and read/write characteristics of FIG. 4F, forexample. Particular access addresses are used to allow these operationsto be performed in one bus transfer (either a read or a write). Specificregister updates (such as writing to the transfer count registers) canbe used to restart a transfer. A list of operations that a hostprocessor can perform follows:

Reset transfer controller;

Write to the INITPC register to place a new address into both TPC andWAITPC;

Write to the TPC register;

Execute a “wait” operation on a semaphore (read SWAIT or UWAIT address);

Execute a “signal” operation on a semaphore (write the INCSx or DECSxaddress, or assert one of the SIGNALSEMx input wires);

Read from the LOCKx register (to acquire a software lock for accessingWAITPC);

Write to the WAITPC to allow instruction processing to advance;

Write to CTC to update transfer count with optional auto-restart;

Write to STC to update transfer count with optional auto-restart; or

Suspend, resume, restart transfers.

The SIGNALSEMx wires provide a set of input signal 465 shown in FIG. 4 ato the transfer controller. These signals are associated with a transfercontroller's semaphore registers 579 shown in FIG. 5E. The EXTSIGregister 990 shown in FIG. 9F is used to configure which of the inputsignals is used to update each semaphore, and to provide an enable bit.A one-cycle pulse on a selected SIGNALSEM signal will cause theassociated semaphore register semaphore to be incremented by 1. If thissignal is asserted on exactly the same cycle that a transfer controlleris executing a WAIT operation on the same semaphore, then the semaphoreis not updated by either operation, and both operations complete as iftheir respective updates occurred sequentially.

An exemplary table 496 of commands and addresses for a presentlypreferred embodiment is shown in FIG. 4F. Two of these commands will bediscussed further, CLEAR 497 and RESTART 498. The CLEAR command may betargeted at both transfer units (CLEAR) or either transfer unitindividually (CLEARSTU, CLEARCTU), and causes a transfer unit toinvalidate its current transfer parameters and enter an INACTIVE state815 illustrated in FIG. 8A. When a transfer unit is in the INACTIVEstate, the only means for getting it back into operation is to fetch atransfer instruction targeted for that unit. The STU has special purposebehavior in this regard, however. When the STU is issued a CLEARSTUcommand and placed in the INACTIVE state, then it becomes a visibleslave on the SDB. This approach means that any data placed into the IDQby an SDB bus master may be distributed to DSP local memories by a CTUtransfer instruction, and any data placed into the ODQ by the CTU can beread from the ODQ by accessing the correct slave address range for thattransfer controller. This behavior is useful for implementing DMA-to-DMAtransfers, as will be discussed further below.

The RESTART command 498 may also be targeted at one or both transferunits (RESTART, RESTARTCTU, RESTARTSTU). When a restart command isreceived by a particular unit, if the unit is not in the INACTIVE state815 shown in FIG. 8A, then the following events occur:

(1) If the transfer count is non-zero, then the transfer unit isrestarted beginning from where it left off, using its current transfercount.

(2) If the transfer count is zero, then the current transfer count isreloaded from the initial transfer count, and the transfer is continuedfrom the address at which it left off

(3) The unit that is not the target of the restart operation willcontinue transferring from where it left off, if its transfer count isnonzero. If its transfer count is zero, then the global CHECKTPC state710 of FIG. 7 will be reentered (or the WAIT state 755, if the restartwas received while in that state).

(4) If both units are targeted with the RESTART, then events (1) and (2)above apply to both units.

A further feature of the RESTART command is the ability to write a newinitial and/or a new current transfer count to a transfer unit togetherwith a RESTART command. Referring to FIG. 4F, writing a count value toINITSTC_START address 499, causes the value to be copied to both the STCand the ISTC (initial STC) registers and a RESTARTSTU 501 is performedalso. Writing a count value to the WRITESTC address will update thecurrent STC, but no restart operation occurs. Using these commands, itis possible to update either or both transfer counts for each transferunit while also initiating an optional restart operation for the unit.

As stated earlier, restart actions can occur either by instruction(RESTART instruction), by command (written to a RESTART address on theSCB, FIG. 4F) or by signal wire, indirectly by updating a semaphore viathe SIGSEMx signals. The transfer restart based on semaphores will bediscussed below.

Transfer controllers can communicate events to host processors using anyof three basic mechanisms: interrupt signals, messages, or semaphores.Each of these mechanisms may be operated in an explicit or an implicitfashion. Explicit operation refers to the operation being carried out bya DMA instruction. Implicit operation refers to the operation beingcarried out in response to an internal event after being programmed todo so. The following sections discuss explicit and implicitsynchronization actions and the instructions or commands associated withthem.

Whenever one of the four internal events “TPC equal to WAITPC”(TPC=WAITPC), “STU end-of-transfer” (STUEOT), “CTU end-of-transfer”(CTUEOT), “STU end-of-transfer and CTU end-of-transfer” (STUEOT&&CTUEOT)becomes TRUE an associated action can be performed if is enabled. Theselection and enabling of these actions is carried out by programmingtwo registers called event-action registers. In a presently preferredembodiment, these registers are designated EAR0 and EAR1 are shown intables 991 and 993 of FIGS. 9G and 9I, respectively. These registers maybe written directly by an SCB bus master or loaded using the LIMEARinstruction.

The EAR0 991 contains flags which enable E0 and E1 event detection andactions. The “E0” flags specify conditions that, when they become TRUE(on each transition from FALSE→TRUE), trigger the corresponding “E0”actions specified in the EAR0 and EAR1 registers. The “E1” flags specifyconditions which, when they become TRUE, trigger the corresponding “E1”actions specified in the EAR0 and EAR1 registers. The “E0” and “E1”conditions are the same so that up to two independent sets of actionsmay be specified for the same event.

This EAR0 register also contains “restart event” fields which allowtransfer restart actions to be triggered automatically when a specifiedsemaphore is non-zero and an EOT condition is reached CTURestartCC,CTURestartSem, STURestartCC, and STURestartSem.

Events are:

CTU reaches EOT condition,

STU reaches EOT condition,

CTU and STU both reach EOT condition (event does not occur unless bothare at EOT), and

When TPC=WAITPC (when this becomes TRUE).

Actions are:

Signal an interrupt using Signal 0 or Signal 1 or both,

Send a message using indirect address and indirect data (Areg and Dregspecifiers),

Update any (or none) of four semaphores by incrementing, decrementing,clearing to zero, and

Trigger a restart event to a specified transfer unit based on the valueof a specified semaphore:

If (RestartCTU is enabled) AND (CTUeot is active) AND (the specifiedsemaphore value is not zero) then the CTU restarts its current transferautomatically (reloading its current transfer count, CTC, from itsinitial transfer count ICTC), and decrements the semaphore atomically.

If (RestartSTU is enabled) AND (STUeot is active) AND (the specifiedsemaphore value is not zero) then the STU restarts its current transferautomatically (reloading its current transfer count, STC, from itsinitial transfer count ISTC), and decrements the semaphore atomically.

Using the above signaling methods, a transfer controller can alert oneor more processors when a specified condition occurs

Interrupt Signals

In a presently preferred embodiment, there are two interrupt signalsavailable to each transfer controller. These may be used as inputs toprocessor interrupt controllers. Explicit assertion of these signals maybe carried out using the SIGNAL instruction 992 of FIG. 9H. Implicitassertion of these signals may be carried out when one of the specifiedinternal events occur by programming the EAR registers shown in FIGS. 9Gand 9I, appropriately either with a host command or through the LIMEARinstruction 493 of FIG. 4C. This latter instruction simply loads the EARregisters with immediate values specified in the instruction.

Message Synchronization

In the presently preferred embodiment, a message is simply a single32-bit write to an address mapped to the SCB, carried out by thetransfer controller. A message requires specification of address anddata. Explicit message generation may be carried out using the SIGNALinstruction with the address, and data may supplied as immediate valuesin the instruction, or with either one or both of address and datavalues coming from transfer controller registers. The GR registers 994of FIG. 9J (see also FIG. 4F for additional details) may be used forstoring both addresses and data for messages. Data values may also comefrom other selected registers such as the TPC, WAITPC, SEM and TSRregisters of FIG. 4F. Implicit message actions are specified in the EARregisters of FIG. 9G and FIG. 9I based on the occurrence of one or moreof the four internal events, and use a specified GR register for theaddress and another register as data (not limited to GR registers).Whenever a specified event becomes TRUE, the programmed message is sent.Several other features of message synchronization are the following.

Since all transfer controllers reside on the SCB, one transfercontroller can synchronize with another through messages to semaphoreupdate addresses, together with WAIT instructions.

A message may not only be a command to another transfer controller, butmay also be an instruction which can be placed into a processor'sinstruction memory. This approach provides a mechanism for synchronizingwith a host processor's execution which does not require eitherinterrupts or polling in the usual sense.

Message capability allows a transfer controller to interact with otherhardware devices on the SCB for simple configuration or controloperation.

Semaphore Synchronization

In the presently preferred embodiment, there are four 8-bit hardwaresemaphores 1066 as illustrated in FIG. 10. Aspects of these semaphoresare also shown in FIG. 5E. The semaphores 1066 may be updated andmonitored by both the transfer controller and host processors in anatomic fashion.

The semaphore registers SEM provide a flexible means for synchronizationof transfers at the intra-transfer (during a transfer) level and at theinter-transfer level (while processing instructions). In addition,semaphores are used as the basis for most conditional operations.Semaphores are located in the SEM registers as seen in FIG. 5E and maybe updated and monitored by both the transfer controller and other busmasters on the SCB in an atomic fashion. The SIGNAL (FIG. 9H) and WAIT(FIG. 10B) instructions 992 and 1082 may be conditionally executed basedon a semaphore value. The SIGNAL instruction may also specify anothersemaphore to update. When a WAIT instruction is executed and thespecified semaphore condition is TRUE, the transfer controller halts thefetch and decode of instructions. When the condition becomes FALSE, theECU decrements the semaphore specified by the WAIT instruction, and thenallows the transfer controller to continue processing instructions.

Another mechanism for semaphore based synchronization makes it possiblefor two host processors to control the data flow during a transferwithout having to communicate directly with each other about dataavailability on the source side, or memory availability on thedestination side. A further feature provided by the EAR registersallows, for each transfer unit, a semaphore to be specified which willcause a transfer to automatically restart if the transfer controller isin the WAIT or CHECKTPC states 755 and 710 of FIG. 7, respectively, andthe transfer unit (STU or CTU) is not INACTIVE 815 as illustrated inFIG. 8A. An exemplary transfer instruction sequence 1083 is shown inFIG. 10C. A host control processor, such as processor 245 of FIG. 2,produces data into a 1K word region of memory in 256 word blocks. Eachof the 4 blocks is written with new data in sequence in a circularfashion (block 0, block 1, block 2, block 3, block 0, . . . etc.). Aproducer block is 256 words. A consumer task running on the DSP 203 ofFIG. 2 has only a 256 word region of memory in which to receive data,split into four 64 word blocks. The DSP processes each of the four64-word blocks in sequence, also in a circular fashion. A consumer blockis 64 words. Every time the producer task finishes filling a buffer, itsignals semaphore S1 by writing to the appropriate command address onthe SCB (INCS1). Whenever the consumer task on the DSP finishesprocessing a buffer, it writes to a command address which increments S0(INCS0). The LIMEAR instruction configures the transfer controller torestart the STU anytime it is IDLE and S1 is non-zero and to restart theCTU any time it is IDLE and S0 is non-zero. When the producer tasksignals S1 the first time, a restart to the STU is initiated. Since theCTU has a non-zero transfer count, then the overall transfer isrestarted and 64 words of data are moved to one of the consumer task'sdata block regions (the minimum of 256 and 64). Every time STU EOToccurs (256 words moved), the transfer controller asserts the signal0interrupt (to the producer task on the host processor) and every timeCTU EOT occurs (64 words moved) a message is sent to an “mbox1” addressfor DSP notification. It is assumed that the producer and consumer taskseach keep track of the data that has been moved using local semaphoresthat are updated based on the signal0 interrupt to the producer task andthe message to the consumer task. Using the code of FIG. 10C, theproducer task is able to generate data at its own rate, and the consumertask processes the data at its own rate. There is no additionalcommunication overhead required between the two tasks.

DMA-to-DMA and DMA—I/O Device Transfers

Each transfer controller supports an SDB-slave address range which maybe used to directly read and write from and to the corresponding ODQ orIDQ when the lane's STU is in an inactive state. For example, a DMAtransfer from SP data memory to PE data memories may be carried out bythe following instruction sequences executed by transfer controller 1and transfer controller 0:

Lane 1:

Clear STU—This makes the STU capable of receiving slave requests for IDQFIFO access.

Transfer instruction—Transfer Core Inbound to PE Data address, “transfercount” words

Lane 0:

Control instruction—setup event-action register to signal interrupt atEOT

Transfer instruction—Transfer Core Outbound from SP Data addresses,“transfer count” words

Transfer instruction—Transfer System Outbound to SDB slave address(es)of Lane 1, “transfer count” words. Lane 1 STU will write data to itsIDQ.

Note that two transfer controllers are used to carry out DMA-DMAtransfers (or one Transfer Controller and another SDB-master).

This same mechanism can be used by any device on the SDB to read/writeto a lane's data queues, allowing one DMA controller or I/O device toread/write data to another. The discussion shows how general “pull” and“push” model DMA-DMA transfers can be implemented.

A “push” model DMA-DMA transfer means that the transfer controller whichis reading the data source acts as the SDB master and writes data to theSDB slave address range of another transfer controller which is writingdata to a destination memory. In this case, the source transfercontroller is executing a TCO, TSO pair of instructions and thedestination transfer controller is executing only a TCI instruction withthe STU inactive (operating as a slave for SDB write access).

A “pull” model DMA-DMA transfer means that the transfer controller whichis writing the data to its destination memory acts as the SDB master andreads data from the SDB slave address range of another transfercontroller which is reading data from a source memory. In this case, thedestination transfer controller is executing a TSI, TCI pair ofinstructions and the source transfer controller is executing only a TCOinstruction with the STU inactive (operating as a slave for SDB writeaccess).

To support a “pull” model DMA-to-DMA or I/O-to-DMA transfer:

Place STU of source DMA into the inactive state (by instruction orcommand).

Program source CTU with an instruction which gathers data from thedesired memories and starts the transfer. This causes the FIFO to befilled but the STU is inactive so that the FIFO will only respond toreads from the source transfer controller's SDB slave port.

Program the destination STU with a TSI.IO instruction using the sourceDMA's SDB slave address as the I/O transfer address to read from.Program the destination CTU with the desired transfer type fordistributing data to destination memories and start the transfer.

The destination DMA Transfer Controller will “pull” data from the sourceDMA transfer controller until either the source or the destinationtransfer unit reaches an end-of-transfer (EOT) condition (the number ofitems transferred is equal to transfer count requested). Semaphores maybe used to make the setup and execution of the transfer almost entirelyoccur in the background.

To support a “push” model DMA-to-DMA or I/O-to-DMA transfer:

Place STU of destination DMA into the inactive state (by instruction orcommand).

Program destination CTU with an instruction which distributes data tothe desired memories and start the transfer. This causes the CTU to waitfor data to arrive in the inbound FIFO. The STU is inactive so that theFIFO will only respond to writes from the source transfer controller'sSTU.

Program the source STU with a TSO.IO instruction using the destinationDMA's SDB slave address as the I/O transfer address to write to. Programthe source CTU with the desired transfer type for gathering data fromsource memories and start the transfer.

The source DMA transfer controller will “push” data into the destinationDMA transfer controller's inbound FIFO until either the source or thedestination transfer unit reaches an end-of-transfer (EOT) condition(items transferred is equal to transfer count requested). Semaphores maybe used to make the setup and execution of the transfer almost entirelyoccur in the background.

Update transfers are special instructions that allow an already loadedtransfer to be updated with a new direction, transfer count or newtarget address (or all three) without affecting other parameters orstate. These types of transfers are useful for minimizing DMAinstruction space when processing transfers that are similar to eachother. An update-type instruction is specified as a variation of a TCI,TSI, TCO or TSO instruction, for example,

tci.update tc=200, addr=0x1000;

The above instruction will update the direction, transfer count andstarting address of a transfer instruction that is already loaded intothe CTU. No other parameters are affected.

The instruction tso.update tc=10 will update only the transfer count ofthe instruction currently loaded into the STU affecting no otherparameters.

Resources Supporting Transfer Synchronization

FIG. 10A shows an ECU (event control unit) 1000 employing event controllogic 1001 and the registers and signals it controls. EAR registers(event-action registers) 1080 specify internal events to be monitoredand corresponding actions to take when they occur. SEM registers 1066support conditional execution of branch instructions and synchronizationinstructions, and may be used to generate restart actions to either ofthe two transfer units when programmed to do so in the EAR registers.They may be updated in three ways: by commands on SCB 1005; by branchand synchronization instructions 1010; and by signals from externaldevices 1015. GR registers 1065 may be used to provide address and datafor synchronization messages to be sent out on the SCB when specifiedinternal events occur. These are specified in EAR registers 1067. Inaddition, the event control logic STU EOT 1030 monitors outputs from theSTU, CTU EOT 1035 from the CTU, the result of the equality comparison ofTPC and WAITPC 1025, and the SCB for commands from an SCB bus master, toupdate, to modify, or to read registers as desired. Based on theprogrammed register values, the ECU 1001 generates interrupt signals toexternal devices 1050, and restart signals to the STU 1055 and CTU 1060.Various other internal control signals 1045 are generated to controlaccess to the SCB and to the registers.

While the present invention is disclosed in a presently preferredcontext, it will be recognized that the teachings of the presentinvention may be variously embodied consistent with the disclosure andclaims. By way of example, the present invention is disclosed inconnection with specific aspects of the ManArray architecture. It willbe recognized that the present teachings may be adapted to other presentand future architectures to which they may be beneficial.

1. A direct memory access (DMA) system comprising: a first transfercontroller coupled to a first memory and to a system data bus (SDB) andconfigured to fetch a first plurality of DMA instructions to operate thefirst transfer controller as an SDB master; and a second transfercontroller coupled to a second memory and to the SDB and configured tofetch a second plurality of DMA instructions to operate the secondtransfer controller as an SDB slave, wherein the SDB master isconfigured to push data read from the first memory to the SDB slave andthe SDB slave is configured to store the data in the second memory. 2.The DMA system of claim 1, wherein the first transfer controllercomprises: a first core transfer unit configured to store the data readfrom the first memory in a first outbound data queue in response to afirst DMA instruction; and a first system transfer unit configured topush the data from the first outbound data queue to the second transfercontroller in response to a second DMA instruction.
 3. The DMA system ofclaim 1, wherein the second transfer controller comprises: a secondsystem transfer unit configured to write the data pushed from the SDBmaster in a second inbound data queue in response to a write operationfrom the SDB master to the SDB slave; and a second core transfer unitconfigured to store the data read from the second inbound data queue inthe second memory in response to a third DMA instruction.
 4. The DMAsystem of claim 3, wherein the second system transfer unit is configuredto be in an inactive state in response to a fourth DMA instruction,wherein the inactive state is responsive to the write operation from theSDB master.
 5. The DMA system of claim 3, wherein the operation ofwriting the data from the SDB master to the second inbound data queue isa push data operation.
 6. The DMA system of claim 1, wherein the firsttransfer controller and the first memory are associated with a firstprocessor and the second transfer controller and the second memory areassociated with a second processor.
 7. The DMA system of claim 1,wherein the first memory is one of a plurality of memories associatedwith an array of processing elements and the second memory is a memoryassociated with a control processor of the array processor.
 8. A directmemory access (DMA) system comprising: a first transfer controllercoupled to a first memory and to a system data bus (SDB) and configuredto fetch a first plurality of DMA instructions to operate the firsttransfer controller as an SDB master; and a second transfer controllercoupled to a second memory and to the SDB and configured to fetch asecond plurality of DMA instructions to operate the second transfercontroller as an SDB slave, wherein the SDB slave is configured to readdata from the second memory, the SDB master is configured to pull thedata from the SDB slave and to store the data to the first memory. 9.The DMA system of claim 8, wherein the first transfer controllercomprises: a first system transfer unit configured to pull the data fromthe second transfer controller into a first inbound data queue inresponse to a first DMA instruction; and a first core transfer unitconfigured to store the data read from the first inbound data queue inthe first memory in response to a second DMA instruction.
 10. The DMAsystem of claim 8, wherein the second transfer controller comprises: asecond core transfer unit configured to store the data read from thesecond memory in a second outbound data queue in response to a third DMAinstruction; and a second system transfer unit configured to read thedata from the second outbound data queue in response to a read operationfrom the SDB master to the SDB slave.
 11. The DMA system of claim 10,wherein the second system transfer unit is configured to be in aninactive state in response to a fourth DMA instruction, wherein theinactive state is responsive to the read operation from the SDB master.12. The DMA system of claim 10, wherein the operation of writing thedata read from the second outbound data queue of the SDB slave in thefirst inbound data queue at the SDB master is a pull data operation. 13.The DMA system of claim 8, wherein the second transfer controller is anI/O device.
 14. A method for direct memory access (DMA) to DMAtransfers, the method comprising: fetching a first plurality of DMAinstructions to operate a first transfer controller as a system data bus(SDB) master, the first transfer controller coupled to a first memoryand to the SDB; and fetching a second plurality of DMA instructions tooperate a second transfer controller as an SDB slave, the secondtransfer controller coupled to a second memory and the SDB, wherein DMAto DMA transfers of data between the first memory and the second memoryare controlled in response to execution of the first plurality of DMAinstructions on the first transfer controller and execution of thesecond plurality of DMA instructions on the second transfer controller.15. The method of claim 14, further comprising: storing data read fromthe first memory in a first outbound data queue in response to executionof a first DMA instruction in the first transfer controller; and pushingthe data from the first outbound data queue to the second transfercontroller in response to execution of a second DMA instruction in thefirst transfer controller.
 16. The method of claim 14, furthercomprising: storing the data pushed from the first transfer controllerto a second inbound data queue in response to a write operation from theSDB master to the SDB slave; and storing the data read from the secondinbound data queue in the second memory in response to execution of athird DMA instruction in the second transfer controller.
 17. The methodof claim 16, further comprising: placing an SDB bus interface unit inthe second transfer controller in an inactive state that is responsiveto the write operation from the SDB master.
 18. The method of claim 14,further comprising: pulling data from the second transfer controller toa first inbound data queue in the first transfer controller in responseto execution of a first DMA instruction; and storing the data read fromthe first inbound data queue in the first memory in response to a secondDMA instruction.
 19. The method of claim 14, further comprising: storingdata read from the second memory in a second outbound data queue in thesecond transfer controller in response to a third DMA instruction; andreading the data from the second outbound data queue in response to aread operation from the SDB master to the SDB slave.
 20. The method ofclaim 19, further comprising: placing an SDB bus interface unit in thesecond transfer controller in an inactive state that is responsive tothe read operation from the SDB master.